This invention relates to detection of specific chemical substances using species selective thin films, planar optical waveguides and laser interferometry.
This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
There is an increasing need for gas sensors which can distinguish extremely low levels of particular gaseous species in the presence of large concentrations of interfering species.
Several fiber optic and integrated optical transducers have been proposed as chemical sensors. Typically, such proposed sensors have been based on a change in the effective refractive index of a guided light mode due to a change in refractive index and/or a change in the thickness of a sensing overlayer. Many such sensors have one or more deficiencies in sensitivity, selectivity, or response time. Thus there is still a need for highly sensitive and selective gas sensors which operate in near real time.
Therefore, it is an object of this invention to provide a method and apparatus for rapid detection of low levels of selected chemical species.
It is another object of this invention to provide a method for sensing chemicals by analysis of changes in the refractive index of a thin film of host reagent on a waveguide caused by the formation of guest-host inclusion complexes in the thin film.
It is yet another object of this invention to provide a method and apparatus for detecting volatile and semi-volatile organic compounds.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The claims appended hereto are intended to cover all changes and modifications within the spirit and scope thereof
Two collinear, orthogonally oriented modes of laser light having a small difference in frequency are transmitted through a planar optical waveguide, into a polarization analyzer and thence into a photodetector such as a photodiode or photomultiplier. The signal from the photodetector is compared with a reference signal from the laser in a phase sensitive detector.
Thin films of host reagents comprised of molecules having cavities are covalently bound to the surface of the waveguide. The host reagents form inclusion complexes with targeted molecules to be sensed.
The refractive index of the thin film of host reagent on the optical waveguide changes according to what species, and in what concentrations, are forming inclusion complexes with the host reagent. The molecules of the targeted chemical species are detected by observing the resulting changes of the effective refractive index of the guided light modes propagating through the waveguide.
The apparatus for chemical sensing comprises: (a) a light source to generate a beam of laser light having two modes of laser light with closely spaced frequencies and that are collinear and orthogonally polarized; (b) a species-selective or size-selective thin film of host reagent bonded to the surface of a waveguide into which the laser beam is directed; (c) coupling lenses to couple light into and out of the waveguide; (f) a polarizer to analyze and transmit a portion of the two light modes emitted from the waveguide; (g) a photodetector to detect the beat frequency generated by interference of the collinear orthogonally polarized modes of light coming from the polarizer; (h) a phase sensitive detector device such as a lock-in amplifier to compare the phase of the photodetector output to the phase of a reference signal supplied by the laser light source. A PC is used to read output and collect data.
Because the formation of inclusion complexes is reversible, the sensor apparatus can be used for substantially real-time sensing of chemical agents. Sensor sensitivity can be tuned and choice of chemicals to be detected can be made by varying the size of the host reagent cavities and by selecting the chemical functionality of the host reagent molecules.
Using a plurality (up to hundreds) of ribbed channels in the waveguide can enhance sensitivity and specificity of the sensor, provide a means for concurrent detection of different chemical species, and provide one or more reference channels. Use of more than one host reagent can enable concurrent detection of more than one species.